1. Field of the Invention
The invention relates to a 3-D camera for recording surface structures on an object of interest by means of triangulation, in particular for dental purposes.
2. Relevant Prior Art
3-D cameras (i.e., cameras for recording three-dimensional structures) for dental applications mostly serve the purpose of recording the surface structure of a tooth in the mouth of a patient. Therefore, they must fulfil several requirements such as the possibility of using the camera “endoscopically” in the mouth of the patient, the possibility of placing the camera in the mouth manually, and a measuring time short enough to avoid blurring even if the camera is used without further fixation. It should further be possible to record the complete surface structure if possible in a single exposure, at a maximum in two exposures. It is therefore important that the results of the measurement are displayed to the operator as shortly as possible after the exposure in order to give him the opportunity to repeat the exposure if necessary. From the resulting 3-D contour data of the surface structure a dental implant will be constructed and produced. The necessary precision of the measurement therefore corresponds to the necessary precision of the dental implant. The maximum gap width that may be tolerated in dental applications is about 100 micrometers (μm). As the steps following the measurement induce further tolerance, a precision of ±25 μm in the relevant measurement volume appears to be a sensible requirement.
U.S. Pat. No. 4,575,805 discloses a 3-D camera with which a surface structure on an object of interest can be recorded in terms of height or depth differences. This conventional 3-D camera has a projection optical path and an observation optical path, which make an angle with an optical axis of the 3-D camera. A light source for emitting a group of light beams in the direction of an object of interest is arranged in the projection optical path. The light reflected by the object of interest is guided through the observation optical path to an image sensor of the 3-D camera. The signals from the image sensor can be fed to an evaluation unit, so that an image of the surface structure can be created on a display device. This 3-D camera is suitable in particular for recording a cavity of a tooth.
EP-A-0 250 993 also discloses such a 3-D camera. For determining the height or depth differences of the surface structure, means are provided for producing a reference pattern in such a way that the reference pattern can be projected onto the surface structure. With the aid of the light which is reflected by the surface structure and is incident on the image sensor, and in conjunction with evaluation electronics for carrying out a process which is referred to as phase-shifting triangulation and is explained in more detail in the aforementioned document, the surface structure can be assessed in terms of height and depth differences and presented as a pseudo-three-dimensional image on a monitor.
A 3-D camera is also disclosed in the journal “Technisches Messen: Sensoren, Geräte, Systeme” [Metrology: sensors, devices, systems], June 1996, pages 254 to 261, Oldenbourg-Verlag B3020.
Although in principle several different triangulation techniques are known, in each of the above-mentioned documents the measurement itself is performed by phase-shifting triangulation. The basic principles of this technique are well-known from the general literature and are described in part in the mentioned documents. In the following a brief introduction is given.
An object is observed by a camera with a planar detector element, which generates a two-dimensional digital image. The object is thus described by a data set in which discrete intensity values are assigned to discrete pixels in the lateral dimension. In order to generate information about the third dimension (object height z), the object is illuminated with incident light in a structured fashion and observed from a direction different from the direction of the incident light, i.e., under a triangulation angle.
The projection and the observation optics must be arranged in a fixed and known spatial relationship to one another. They may be formed by the same system of lenses which are transmitted in different areas or under different angles. For influencing the beam geometry, an additional field lens may be present close to the object.
The illumination structure is generally periodic in one dimension and homogeneous in the other dimension, i.e., it is a strip pattern (line pattern). For determining the height values of the third dimension, this line pattern is moved across the object, and during this moving operation at least three, mostly four or five images are recorded.
The application of phase-shifting triangulation to recording dental structures is characterized by a number of specific demands. The most important ones are imposed by the size and typical shape of the object and by the necessary precision. For measuring edges with height differences of up to about 10 millimeters (mm) with a precision of 25 μm in all dimensions, a high lateral resolution of the optical components is necessary. The simultaneous need for a high depth of field imposes demands to the system that are at the edge of what is possible with visible light because of the diffraction limit. A short wavelength would be desirable if suitable light sources were available.
The demand of recording cavities with steep walls on all sides requires a small triangulation angle. The small object size allows to use a telecentric beam path, which leads to simplifications in the evaluation algorithms. A telecentric beam path also is a good compromise with respect to the shape to be measured, as it allows measuring cavities as well as stumps with steep flanges.
There are several techniques by which the actual measurement may be performed with a 3-D camera. One of these is described in the above-mentioned U.S. Pat. No. 4,575,805. The basic approach is to take four images at different positions of the line pattern (ruling) with respect to the object. Between these images the line pattern is shifted by an amount corresponding to a phase shift of 90° with respect to the periodicity of the pattern. These images are then used for calculating the height profile. This is done by first taking the differences between the image for 0° phase shift and the image for 180° phase shift, and between the 90° and 270° images, respectively. The first difference is called the 0°-180° image, the second the 90°-270° image. For any given pixel, the intensity values of these difference images can be shown to correspond to the real and imaginary parts, respectively, of a complex number. The complex phase of this number is then proportional to the height value of the corresponding pixel with respect to a fixed reference height.
A slightly different technique is proposed in the above-mentioned article in the journal “Technisches Messen: Sensoren, Geräte, Systeme”. There, the line pattern is moved continuously across the object while the images are taken. The detector integrates the actual intensity in each pixel over a certain time span, e.g. {fraction (1/30)} sec. with an inter-line CCD which is operated according to the NTSC norm. If the velocity of the moving pattern is chosen in such a way that during the integration period of four images the pattern is shifted exactly by one period (360°), and if four continuous images are acquired, the height profile can be calculated in a similar manner as for four images with a static, phase-shifted pattern.
The line pattern (reference pattern) can be produced, e.g., by a mechanical grating or by an LCD arrangement in the projected beam. With a mechanical grating, the movement of the line pattern can be achieved by moving the grating, e.g., via a coil-and-plunger construction or via a piezo actuator. If the line pattern is produced by an LCD element, the movement can be generated electronically by applying appropriate electronic signals to it.
For a given period of the reference pattern, there is an unambiguous range, i.e., the range in which the height difference between two object points can be unambiguously recorded, according to the following formula:
Unambiguous range=period of the reference pattern divided by the tangent of the angle which the projection optical path and the observation optical path make with one another.
Limited by electrical noise and other effects, the achievable measurement accuracy is always some fraction of the unambiguous range (typically {fraction (1/100)}). Consequently, for a large period the unambiguous range is large, although the height difference between two object points cannot be recorded so accurately. For a small period, the unambiguous range is small but the height difference between two object points can be recorded with great accuracy.
Since it is desirable to be able to record even large height differences between two object points unambiguously and accurately, a 3-D camera has been proposed in DE 90 13 454 U1 in which means for producing a first reference pattern and a second reference pattern on the object of interest are present in the projection optical path. By projecting reference patterns with preferentially different periods onto the object of interest, a substantially larger height difference between two object points can be recorded unambiguously compared with the use of only one reference pattern.
A disadvantage with this that either superposition of the first grating on the second is necessary, with the result that poorer measurement accuracy is achieved on the whole, or a long recording time is necessary. On the whole, the design outlay is very high.
Even in the case of unfavourable surface structures, in order to make measurement of the surface structure possible here, further means for producing a further group of light beams are proposed, which can be guided onto the object of interest from a second direction, different from the first, via a further projection optical path. As a result, the surface structure can be illuminated from different directions, it being proposed that a means for producing a reference pattern be arranged in each projection optical path.
A disadvantage with this is that the equipment outlay is large precisely for manually operated 3-D cameras, and a device which is easy to handle can therefore only be produced with difficulty.
This disadvantage also arises with WO 98/11 403 A1, which discloses a process and a device for the three-dimensional measurement of objects by optical recording, projected patterns and triangulation calculations, in which the projection unit for the pattern and the recording unit are constructed separately from one another and can be positioned or introduced in the course of the measurement process independently of one another.